I read something similar to this a couple of years ago. It was written by a self-professed advocate of public transit - and an urban planning armchair quarterback. He did a decent analysis of the energy inputs for various forms of transit. Similar to P-MPG, it was measured in P-BTU's per mile and found that public transit wasn't as efficient as he thought it would be. Naturally, ridership in the US isn't what it could be, so as ridership goes up, so too would the P-BTU metric.